Characteristics of Living Organisms

MRS GREN is useful for remembering the seven names, but the exam doesn’t reward the mnemonic — it rewards the precise definition behind each letter. An organism also doesn’t have to visibly show every characteristic at every single moment to count as living. A dormant seed can look completely inactive for a long time and still be alive, because it remains capable of resuming active life processes once conditions become suitable.

Movement

Movement is an action by an organism, or part of an organism, causing a change in position or place.That definition doesn’t require the whole organism to travel anywhere. An animal walking across the ground changes the position of its entire body, which is the obvious case. But a plant normally stays rooted in one spot, and its parts can still move — a shoot growing towards a light source is changing its position even though the whole plant never leaves the ground. So movement can mean movement of the whole organism, or movement of just one part of it. The idea that has to survive into your answer either way is change in position or place.

Respiration

Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. It happens inside cells, and the energy it releases becomes available for movement, active transport, growth, cell division, building larger molecules, and maintaining suitable internal conditions. Metabolism is simply the name for the whole collection of chemical reactions taking place in an organism.

This is the characteristic examiners catch students out on most, because respiration is not breathing. Breathing is the physical movement of air into and out of gas-exchange organs such as the lungs. Respiration is the set of chemical reactions happening inside cells. Breathing can supply the oxygen that aerobic respiration uses, but breathing itself releases no energy from nutrient molecules — only respiration does that. Hold onto the relationship nutrient molecules are broken down in cells → energy is released for metabolism, and you won’t reach for “breathing” when a question is really asking about respiration. Aerobic and anaerobic respiration are studied in much greater depth later in the course.

Sensitivity

Sensitivity is the ability to detect and respond to changes in the internal or external environment. A detectable change is called a stimulus, and external stimuli include light, sound, temperature, touch and chemicals — but changes can also happen inside an organism, such as a shift in body temperature or in the concentration of substances in the blood. The characteristic always involves two connected steps: detect a change → produce a response. Bright light detected by the eye leading to a smaller pupil is the classic example. Plants show sensitivity too — a shoot receiving light mainly from one direction can grow towards it — and the response doesn’t have to be fast. A plant’s response can unfold gradually, through growth, and still count.

Growth

Growth is a permanent increase in size and dry mass. Both halves of that definition matter, and each one rules out a common wrong answer. Growth has to be permanent— a stomach getting bigger after a meal is a temporary change, not growth, because the organism itself hasn’t grown. And growth involves dry mass, the mass of biological material once water has been removed, because an organism can get heavier simply by absorbing water. A dry seed placed in water gains mass immediately, but that alone doesn’t prove the seed has grown — the extra mass might just be water. True biological growth is a lasting increase in size and in the amount of biological material present, which is exactly why dry mass, rather than total mass, is the measure that matters here.

Reproduction

Reproduction is the processes that make more of the same kind of organism.It’s what lets a population continue from one generation to the next, and later in the course you’ll study its two major forms in detail: sexual reproduction and asexual reproduction. An individual organism doesn’t need to reproduce at every stage of its life to count as living — a young organism may not yet be able to reproduce, and some individuals are infertile. Reproduction still counts as a characteristic of that kind of organism, because organisms of that type are capable of producing new individuals.

Excretion

Excretion is the removal of waste products of metabolism and substances in excess of requirements. Cells are constantly carrying out chemical reactions, and some of those reactions produce substances the organism no longer needs — others are useful in small amounts but harmful if they build up. Carbon dioxide from respiration, urea from the breakdown of excess amino acids in humans, and excess water or mineral ions are all examples. Left to accumulate, these substances interfere with normal body processes.

The distinction Cambridge tests directly is excretion versus egestion. Excretion removes metabolic waste products and substances present in excess. Egestion removes undigested food from the alimentary canal — food that was never absorbed into cells in the first place, and so was never a waste product of metabolism at all. That means the removal of faeces is egestion, not excretion, even though it’s tempting to lump the two together as “getting rid of waste.”

Nutrition

Nutrition is the taking in of materials for energy, growth and development.Every organism needs materials from its environment, but not every organism gets them the same way. Animals obtain organic nutrients from other organisms or from material those organisms have produced — nutrients that supply materials for growth and repair, substances for building new molecules, and nutrient molecules that can be broken down in respiration. Plants instead manufacture organic nutrients such as glucose through photosynthesis, taking in carbon dioxide and water and absorbing mineral ions from the soil. Nutrition, in other words, means more than simply “eating” — it means obtaining whatever materials are needed to provide energy and support growth and development, however an organism does that.

How the seven characteristics connect

The seven characteristics aren’t seven unrelated facts to memorise separately — they describe different parts of one working system. Nutrition provides the materials. Respiration releases energy from those materials for metabolism. That energy and those materials support processes including growth and movement. Sensitivity lets an organism detect changes and respond to them. Metabolic reactions produce substances that may need removing through excretion. Reproduction produces new organisms of the same kind. None of this runs as one fixed sequence — the processes interact continuously as part of being alive — but seeing the connections is what stops MRS GREN from being seven isolated definitions and turns it into one coherent picture of what “living” actually means.